High voltage multi-layer cylindrical devices

ABSTRACT

High voltage cylindrical devices having coaxial cylindrical insulation layers of different strength constants, the strength constants and the dimensions of the insulation layers being selected to achieve approximately the minimum obtainable outside diameter of the device or to achieve a device having a smaller outside diameter than a one layer device of equivalent electrical characteristics.

United States Patent [191 Denes 1 HIGH VOLTAGE MULTl-LAYER CYLINDRICALDEVICES 1 Jan. 16,1973

9/1967 Cox ..3l7/259X 3/1969 Zysk ..l74/l20R FOREIGN PATENTS ORAPPLlCATlONS 223,198 7/2959 Australia ..l74/1 10 PM 1,568 1901 GreatBritain ..174/l20 R Primary ExaminerE. A. Goldberg Attorney-Richard A.Bachand [5 7] ABSTRACT High voltage cylindrical devices having coaxialcylindrical insulation layers of different strength constants, thestrength constants and the dimensions of the insulation layers beingselected to achieve approximately the minimum obtainable outsidediameter of the device or to achieve a device having a smaller outsidediameter than a one layer device of equivalent electricalcharacteristics.

8 Claims, 2 Drawing Figures DIELECTRIC LAYERS PATENTEDJAHIB I9753.711.631

FIG 2 INVENTOR. PETER A. DENES BY ATTORNEY HIGH VOLTAGE MULTI-LAYERCYLINDRICAL DEVICES BACKGROUND OF THE INVENTION smaller outside diameterthan single layer devices of equivalent electrical characteristics, thediameter approaching the optimum minimum obtainable, and to a processfor making same.

2. Description of the Prior Art In single layer high voltage cylindricaldevices, such as high voltage power transmitting cables, tubularcapacitors, and the like, the electrical field created in the device hasa hyperbolic distribution, with the highest electrical stress appearingon the inside surface. In such devices constructed with insulatingmaterials of presently known substances the outside diameter of a singlelayer insulation cable is generally impractically large at highervoltages. Thus, as a practical matter, in the past such cylindricaldevices have been unusable for power transmission and other suchdesirable uses.

As an example, in a single insulation layer cable with a working voltageof 670 kv RMS, 60 Hz alternating voltage, the peak voltage on the cableis 950 kv. If the radius of an inner copper conductor, coated with anequipotentializing semi-conductive layer, is R, 1 cm, and the insulationof the cable is extruded cross-linked polyethylene having an allowabledielectrical stress of E 200 kv/cm, the outside radius, R,,, will be 115cm, or, an outside diameter of more than 90 inches. This would be notonly too expensive, but practically, impossible.

In the past, cables with multi-layer insulation have been proposed forvarious purposes, and many improvements of mechanical, environmental,and other aspects, not herein pertinent, have been made. Theimprovements in many multi-layer applications are employed forincreasing the mechanical strength, flexibility, humidity resistance,and the like, of the systems, but the use per se of multi-insulationlayers in the prior art devices serves no particular electrical purpose.

For example, in one multi-layer system advanced, semi-conducting layerson the conductor and metal shield are used to eliminate corona on thesurface of the conductors. Although the electrical characteristics ofthis device are of interest, and although such arrangements may also beused with cables in accordance with the present invention, they areunrelated to minimizing the outside diameter of the cable, the subjectmatter of the present invention.

Propositions have also been made for improving voltage carrying cablesin which several layers of insulating materials are arranged with theinner insulations having a higher breakdown strength than the outerinsulation, taking the hyperbolic stress distribution intoconsideration. This structure, however, has no particular technicaladvantages except possibly a cost advantage over a cable with a singleinsulating layer of highest breakdown voltage material.

In another proposal, disclosed in U.S. Pat. No. 3,433,891, severallayers of crosslinked ethylene polymer containing various amounts oftitanium dioxide are employed to achieve varying dielectric constants ofthe layers, increasingly inwardly. Again, although a structure made inaccordance with this proposal results in decreased outside cablediameter, the decrease is generally insignificant since the method bywhich it is achieved fails to recognize the determination of the optimumradii, dielectric, and strength constants of the different layers. Infact, based on equations disclosed below as a part of the presentinvention using the data of the most effective example of U.S. Pat. No.3,433,891: three layers having dielectric constants 4.8, 3.2, and 2.4;with the innermost layer comprising 15 percent, the intermediate layer30 percent, and the outermost layer percent of the total wall thickness;and the allowable dielectric stress being 200 kv/cm, the cable diameteris more than 27 inches. This, of course, is too large and expensive forpractical considerations.

No prior art recognized the decisive importance of determining theapproximate correlation between the ratios of dielectric constants, or,more precisely, the ratios of the strength constants of thesubsequential insulating layers and the respective dimensions of theinsulating layers to obtain the minimum possible outside diameters forsuch high voltage cylindrical devices, as, for example, in powertransmission cables, capacitors, insulators, and the like. (Strengthconstant is a new characteristic of insulating materials introduced inthis disclosure, as described in detail below, and is defined as theproduct of the dielectric constant and the maximum allowable dielectricstress.) Consequently, because of the difficulty in obtaining practicalcable dimensions, for instance, power line transmission cables in usetoday rarely carry voltages about 345 kv RMS, and, in general, even mostlower voltage carrying power transmission lines are the commonly seenaerial lines. Aerial lines have many environmental and ecologicaldisadvantages; among others, they are prone to atmospheric damages,represent dangers to low flying airplanes, and have more power outingsthan cables. It is to these disadvantages, as well as others discussedbelow, that the present invention is addressed.

SUMMARY OF THE INVENTION In light of the above, therefore, it is anobject of the invention to provide a cylindrical multi-layer electricaldevice having the approximate minimum obtainable outside diameter, andmethod for constructing such device.

It is another object of the invention to provide a cylindricalmulti-layer electrical device having a smaller outside diameter than asingle layer electrical device of substantially equivalent electricalcharacteristics, and method for constructing such device.

It is another object of the invention to provide a cylindricalmulti-layer electrical transmission line, and a method for making same,having an approximate minimum obtainable outside diameter.

It is still another object of the invention to provide a cylindricalcapacitor, and method for making same, having the approximate minimumobtainable outside diameter.

It is still a further object of the invention to provide a cylindricalmulti-layer device, and method for making same, in accordance with theforegoing objects, which has particular high voltage applications.

It is yet another object of the invention to present a practicallyemployable high voltage transmission cable which overcomes many of thedisadvantage of heretofore used aerial transmission lines.

These and other objects, features, and advantages will become apparentto those skilled in the art from the accompanying drawing and appendedclaims, read in conjunction with the following detailed description.

In accordance with the present invention, a first method for fabricatinga multi-layered electrical device, and the resulting structure having asmaller outside diameter than a one layer device of equivalentelectrical characteristics are presented. In the first method,insulating materials with steadily increasing strength constant valuesM,, M, are selected. Then, the insulating materials are formed about aninner conductor, the radius of any one of the insulating layersdeviating less than 50 percent downwards and +250 percent upwards fromthe values of the radii R R, defined by:

and

Additionally presented is a second method for fabricating a cylindricalmulti-layer electrical device having approximately the theoreticalminimum obtainable outside diameter, and the resulting structure. Inaccordance with this second method, the device is achieved by selectingthe number of layers' in which the breakdown strength and strengthconstants of one insulating material are known, and the approximatevalues of allowable dielectric stress of the rest of the layers areknown. Then, for each of the selected number of layers available andtechnically employable, insulating materials are selected the strengthconstants of which are close to the theoretically optimum strengthconstants defined by the following formulas, in which the asteriskdenotes theoretical values:

radii R* R, are those of the theoretically minimum outside radius R*,,,defined by 1 In i, l 1 i i l (1. Z, ,n),

and

Then, the selected insulating materials are formed about an innerconductor in accordance with the above described first method forfabricating a device having a smaller outside diameter than a one layerdevice of equivalent electrical characteristics.

DESCRIPTION OF THE DRAWING In light of the above, the invention isillustrated in the accompanying drawing, wherein:

FIG. 1 shows a cross-sectional end view of a multi- DESCRIPTION OF THEPREFERRED EMBODIMENT A preferred embodiment of a cylindrical highvoltage multi-layer device and method for constructing it, in accordancewith the invention, are described with beginning reference to thefollowing derivations of the enabling equations. With specific referenceto FIG. 1, an n-layer cylindrical device, generally denoted by referencenumeral 10, is illustrated having a central conductor 11 with radius Rand having n insulating layers with outside radii of R R,,. The dottedlines 12 illustrate that the number n may be a number greater or equalto 2. The dielectric constants of the respective n layers are q, ,e,,,and the maximum allowable dielectric stresses in the n layers are E,,,E,,.

The capacitances of the n insulating layers are where K is a constant,incorporating the length of the device.

The maximum electrical stress of the i-th insulating tube appears on theequipotential surface of radius R and its value is where V is a voltagedifference between the equipotential surfaces of radii R, and R Sincethe electrical charge, Q, is the same throughout each equipotentialsurface:

Q C V,= constant (1' l,. ,n); hence, from the above equations follows:

E R constant (i= 1, ,n). (3)

In equation (3), the term M is introduced to represent the strengthconstant of the material, and is defined as M Ee, with dimensions ofvolts per cm. (In the calculations below, M is expressed in kv/cm.)

From equation (3) follows:

M R, =constant (i l,. ,n). As appears more fully below, the strengthconstant is a particularly important parameter of insulating materialsemployed in multi-layer cylindrical devices. For convenience, thestrength constant of some commonly Sulfur hexafluoride gas, 3 atmPolypropylene 2 Polyethylene 2.4 200 480 Polycarbonate 3 I50 450Polyethylene terephthalate 3.1 300 930 Vinylidene Chloride VinylChloride Copolymer 4 300 I200 Polyvinyl Fluoride 8.5 250 2120 Thedetermination of the values of radii R ,R,,,

which yield the smallest outside radius, R,,, is of great importance andis our next step.

As the sum of V ,V,, equals V:

the maximum sustainable voltage results for a device if all thefollowing partial differential quotients are equal to 028/8R, V=(i= 1,.,n-l),

Substituting the natural logarithm expressions from equation (6) inequation the n-th equation results as:

V=E,R ln(R,/R )E,R +E,,R,, (7) Equations (6) and (7) define the radiiR,, R,, which yield the highest sustainable voltage, or, at the sametime, if V and the stresses E,, ,E,, are given, the same equationsdetermine the smallest possible radii of the different insulatinglayers. Substituting these radii values in equation (4), the optimumstrength constant of the n layers can be readily determined.

In the following examples, multi-layer variations of the above describedone layer cable are presented, illustrating the principles in accordancewith the invention. For the sake of easy comparison the basicparameters: V=950 kv and R lcm are considered in all examples.

EXAMPLE I In this example, two insulating layers are employed and theoptimum conditions are sought. The allowable stresses of the two layersare approximated as E,=300 kv/cm and E =200 kv/cm.

Equations (6) and (7) result in R,=3.75 cm and R =6.8 cm. If the outsideinsulating layer is crosslinked polypropylene, the strength constant ofwhich is M =40O kv/cm, then from equation (4) results M =l5 00 kv/cm.The smallest cable dimension is, therefore, obtainable with aninsulating material having the strength constant.

EXAMPLE 2 In the same high voltage cable, three different insulatinglayers are employed which have the allowable dielectric stresses ofE,=300 kv/cm, E =250 kv/cm and E =200 kv/cm. The following optimum radiiare obtained from equations (6) and (7): R,=2.72 cm, R =4. 75 cm and R=6.3 cm. If the outside layer is again polypropylene, M =400 kv/cm. Fromequation (4) it follows that M,=l ,890 kv/cm and M =7O0 kv/cm.

Of course, insulating materials with strength constants are obtainedfrom these calculations may not always be available, or, even ifavailable, other parameters of the available material may beunacceptable, for example, the power factor, temperature resistance, andso forth. In such cases the insulating materials have to be chosen fromthose having strength constants approximating the optimum strengthconstant values.

In most situations, it is advantageous to compare the originallysupposed E values with those of the actually selected materials. If adeviation of more than 50 percent exists, it is recommended that R,,,R,, and M ,M,, be redetermined from the corrected equations (6) and (7)to gain the corrected optimum values for M,, ,M,,. In some cases, infact, the redetermined values may facilitate the selection of moresuitable insulating materials.

In the following two examples, the optimum strength constants obtainedin Examples 1 and 2 are substituted with data of actually availablematerials.

EXAMPLE 3 From Table I, the following materials have the closeststrength constants to the optimum ones in Example 1:

MATERIAL l: Vinylidene chloride vinyl chloride copolymer, M =l ,200kv/cm;

MATERIAL 2: Crosslinked polypropylene, M =400 kv/cm. From equations l(4) and (5), the radii result as R,=3 cm and R =8.5 cm. The diameter ofthe cable, then, is approximately 7.5 inches.

EXAMPLE 4 From Table I, the following materials have the closeststrength constants to the optimum ones in Example 2:

MATERIAL l: Polyvinyl fluoride, M ,=2,l20 kv/cm; MATERIAL 2: Vinylidenechloride Vinyl chloride copolymer, M =l ,200 kv/cm;

MATERIAL 3: Cross-linked polypropylene, M =400 kv/cm. Equations (1), (4)and (5) result in R,=l.77 cm, R =5.3 cm and R =6.5 cm. The outsidediameter of the cable, thus, is approximately 6 inches.

Clearly, many more insulating materials having high strength constantsare available besides those listed in Table l. The dielectric constantof an insulating material can also be regulated by adding other organicmaterials or inorganic pigments to it. In the latter case care must beexerted that the dielectric constant of the pigment not be much higherthan that of the organic polymer because the dielectric stresses at thepigment polymer interfaces are inversely proportional with thedielectric constants of the two materials, thereby lowering theallowable stress for the system. The loss in the stress may be largerthan the gain in the dielectric constant, and the result may be a dropof the strength constant, in spite of the increase of the dielectricconstant.

Since the losses of the high voltage transmission cables areconsiderably important, the selection of the insulating materials shouldnot be governed only by the strength constant values, but thedesirability of using materials having low power factors should also beconsidered. (This may, however, necessitate some sacrifice in theoptimum values of strength constants.)

Another consideration in the manufacture of multilayer high voltagedevices in which, in a particular application, the layers need not beany specified critical radii, is that sometimes it may nevertheless benecessary to deviate from the predetermined optimum dimensions. Oneimportant reason for this may be to diminish the voltage difference on acertain insulation section having a relatively higher loss factor. Otherless impor tantreasons may be facilitation of production, increase ofstrength, control of flexibility of a cable, and the like. In thisregard, it has been found that a deviation between 50 percent and +250percent in the inner radii generally does not exceedingly increase theoutside diameter of the device, and in most cases is allowable.

In the previous examples, the losses of the cable were not considered.In the following examples, when the different insulating materials areselected, the power factors of the cables are also taken intoconsideration.

EXAMPLE In Example 3, the first insulating layer is changed topolyethylene terephthalate containing 30 percent by weight fine aluminapowder. (Alumina was chosen because its dielectric constant is not toohigh and its power factor is quite low.) The parameters of this materialare: e,=3.5, E =300 kv/cm, M,=l,050 kv/cm, P,=0.0025, P denoting thepower factor. The second layer is cross-linked polypropylene, e =2,E5200 kv/cm, M =40O kv/cm, P =0.0003.

Equations (1), (4) and (5) result in R,=2.6 cm and R =9.5 cm. Thediameter of the cable, therefore, is approximately 8 inches.

If the frequency is 60 Hz, the losses of the i-th section of the cableis in watts per meter cable length:

EXAMPLE 6 The losses of the cable described in Example 5 may still betoo high for usage in some applications. In this example, the methodwill be shown by which the losses of the cable are decreased, even ifthe same materials are employed, by diminishing R whereby the voltagedifference on the lossier layer becomes smaller.

The materials are identical with those of Example 5 but R =2 cm isselected (allowable because the decrease from 2.6 cm is less than 50percent). Equations (1), (4) and (5) give: R,=I4.6 cm. The diameter ofthe cable is approximately 12 inches.

In this example, U =l l0 kv RMS, U,=560 kv RMS. The losses are: W,=307w/m and W =I86 w/m. The total losses, 493 w/m, are approximately half ofthe losses of the cable according to Example 5. However, the diameter ofthe less lossy structure is 50 percent larger.

EXAMPLE 7 The losses can be further diminished by selecting material 1to have a lower power factor. Cross-linked polyethylene with 60 percentby weight alumina powder filling has the following parameters: e,=3.8,E,=200 kv/cm, M =760 kv/cm, P =0.0004. Equations (1), (4) and (5) give:R =l.9 cm and R,=l7.4 cm. The voltage and loss distributions are: U-,=9Okv RMS, U,=580 kv RMS, W,=38 w/m, W =l84 w/m. The total loss of thecable is only 222 w/m; however, the diameter of the cable isapproximately 14 inches.

EXAMPLE 8 In this example, sulfur hexafluoride gas is employed under apressure of 3 atm as the second insulating layer: e =l, E =l50 kv/cm.Supposing E =200 kv/cm, equations (6) and (7) give R,=5.4 cm and R =l1.5 cm, and equation (4) gives e,==4.05.

In the actual realization of the cable, the inside insulating layer iscross-linked polypropylene with 60 percent by weight alumina powderfilling which has the following parameters: e,=3.6, E =200 kv/cm, M =720kv/cm and P,=0.0004. Equations (1), (4) and (5) result in R,=4.8 cm andR =1l.6 cm. The voltage across the first layer is U ,=224 kv RMS and itsloss is W,=l 15 w/m. This is approximately the total loss of the cable,because losses in the gas layer are negligible. The diameter of thecable is approximately 11 inches, including the gas-tight shield-tubing.

In Table II, the dimensions of the various discussed cables arecompared. Comparing the volumes, the minimum volume of Example 4 is setas the unit volume.

TABLE II Cable System Outside diameter Volume in inches Ratio One-layercable 225 Three layers according to U,.S. Pat. No. 3,433,891 27 20Example 3 7.5 L6 Example 4 6 1 Example 5 8 1.8 Example 6 l2 4 Example 7l4 5.4 Example 8 l l 3.4

Table II shows the great volume gains with cables made according to thisinvention, even if the losses are kept to very low values.

Multi-layer cables can be produced by many methods known in the art; forexample, by a multiple extrusion, or by wrapping the different layersusing ribbon type insulators. Whatever method is employed, it isimportant that no air enclosures be in any of the layers or betweenadjacent layers.

In the previous examples, high voltage cables were mainly considered.Another important high voltage cylindrical device is a high voltagetubular ceramic capacitor. An example of a three layer ceramic capacitorwill be disclosed, compared to a one layer capacitor, because ceramicmaterials differ from organic polymers in some respects; for instance,the allowable dielectric stress in ceramic materials is generally muchlower, averaging E==20 kv/cm. The dielectric constants of ceramics varywithin much broader limits, for example, from 2 to 20,000. Practically,almost any dielectric constant can be created between these limits. Forhigh voltage applications, the limits of dielectric constants of usefulceramic compositions are between 3 and 3,000, today.

The following comparative examples discuss ceramic capacitors, and, forease of description, the following specifications are assumed throughoutthe examples: inside radius, R =0.2 cm; the length of the electrodes 1cm; the capacitance, 200 pf; and the peak working voltage, kv. All theemployed ceramic layers have an allowable dielectric stress of E=20kv/cm.

If only one dielectric layer is employed, the outside radius, inaccordance with the above formulas, is 29.4 cm, again an impossibledimension. The dielectric constant of the ceramic composition should be1,800 to yield the required capacitance.

EXAMPLE 9 In the ceramic capacitor of this example three differentceramic layers are used. Equations (6) and (7) give an outside radius ofonly L056 cm, which is approximately 3 percent of the radius needed inthe onelayer capacitor. The volume of the ceramic capacitor madeaccording to the invention has a volume of less than one tenth of apercent of the volume of the onelayer capacitor.

The dielectric constants of the three layers which give a capacitance of200 pf are: 6 1 ,160, s =570 and e =335. These point out a furtheradvantage of the capacitor made in accordance with this invention,namely, that ceramic compositions of lower dielectric constants are onlyneeded. Generally, ceramic compositions of lower dielectric constantscan be selected to have higher breakdown voltage, lower temperaturedependence of the dielectric constant, higher volume resistivity, andlower ageing, compared to the very high dielectric constant ceramiccompositions.

Multi-tubular high voltage ceramic capacitors according to the inventioncan be made by several methods. They can be, for example, subsequentlydipped, employing the methods of US. Pat. No. 3,016,597, and thencofired. During firing, a limited codiffusion takes place between thelayers of different compositions which, if similar compositions areused, does not unacceptably alter capacitance value. The codifficusioncan be further limited by inhibiting layers at the separation surfaces.

In another version, the ceramic tubes of the high voltage capacitor canbe formed and fired individually. FIG. 2 shows a multi-layer ceramiccapacitor which is assembled of separate tubes. To avoid air layersbetween the ceramic tubes, all tubes have an inside and outsideelectrode. in a feed-through type capacitor, as denoted by referencenumeral 20 in FIG. 2, the inside electrode 21 of the innermost ceramictube 22 goes through the full length of the tube. All other electrodesoccupy only the center part of the tubes, leaving sufficient insulatingareas to carry the high voltage imposed between the inside electrode 21and outside electrode 23.

The inner electrodes are floating. All the electrodes are coated with asoldering or brazing material, to unite the inner electrodesmechanically and electrically.

The disclosure showed in a few examples the great reduction ofdimensions of high voltage cylindrical devices made employing theprinciples of the invention. The applicability of the principles of theinvention is not limited to the examples and many other types of highvoltage cylindrical devices, such as, for example, high voltageinsulators, and so forth, can be created using the new recognitions ofthis invention. The selection of the insulating materials is notlimitedto the used examples either; many other types of solid, liquid, gaseousor complex insulating materials can be employed the parameters of whichsatisfy the conditions of this invention.

Although the invention has been described and illustrated with a certaindegree of particularity, it is to be understood that the presentdisclosure is made by way of example only, and that numerous changes andmodifications will appear to those skilled in the art which fall withinthe scope of the invention, as hereinafter claimed.

What is claimed is:

1. A cylindrical electrical device, comprising: a cylindrical innerconductor having a radius R a plurality n of cylindrical layers ofinsulating material formed about said inner conductor and each having anoutside radius R,, a maximum allowable dielectric stress E, and astrength constant M, equal to the product of the dielectric constantthereof and the maximum allowable dielectric stress E, thereof, 1' beingthe identification number of the layer and being unity for the innermostlayer and n for the outermost layer, said device being operable with apeak voltage difference V between said inner conductor and the outersurface of the outermost layer of insulating material, the product ofthe strength constant M, and the inside radius of the first layer beingapproximately the same as the product of the strength constant andinside radius of each of the other layers, and the respective radii ofsaid layers being approximately equal to the radii required to satisfythe following equations: 1

application of said peak voltage V to said device being effective tosimultaneously create said maximum allowable stresses E, in therespective layers.

2. The device of claim 1 wherein said device is a capacitor.

3. The device of claim 1 wherein said device is an insulator.

4. The device of claim 1 wherein said device is a cable.

5. The device of claim 4 wherein the outermost insulating layer is agas.

6. The device of claim 4 wherein the material of one insulating layer isselected from the group consisting of aluminum oxide powder filledpolypropylene and aluminum oxide powder filled polyethylene.

7. The device in accordance with claim 1 wherein the first insulatinglayer is of cross-linked polypropylene mixed with alumina powder and thesecond insulating layer is of sulfur hexafluoride gas under pressuregreater than 1 atm.

8. The device in accordance with claim 1 wherein the first insulatinglayer is of cross-linked polyethylene mixed with alumina powder and thesecond insulating layer is of sulfur hexafluoride gas under pressuregreater than 1 atm.

II! Ik

1. A cylindrical electrical device, comprising: a cylindrical innerconductor having a radius RO, a plurality n of cylindrical layers ofinsulating material formed about said inner conductor and each having anoutside radius R1, a maximum allowable dielectric stress Ei and astrength constant Mi equal to the product of the dielectric constantthereof and the maximum allowable dielectric stress Ei thereof, i beingthe identification number of the layer and being unity for the innermostlayer and n for the outermost layer, said device being operable with apeak voltage difference V between said inner conductor and The outersurface of the outermost layer of insulating material, the product ofthe strength constant M1 and the inside radius of the first layer beingapproximately the same as the product of the strength constant andinside radius of each of the other layers, and the respective radii ofsaid layers being approximately equal to the radii required to satisfythe following equations: application of said peak voltage V to saiddevice being effective to simultaneously create said maximum allowablestresses Ei in the respective layers.
 2. The device of claim 1 whereinsaid device is a capacitor.
 3. The device of claim 1 wherein said deviceis an insulator.
 4. The device of claim 1 wherein said device is acable.
 5. The device of claim 4 wherein the outermost insulating layeris a gas.
 6. The device of claim 4 wherein the material of oneinsulating layer is selected from the group consisting of aluminum oxidepowder filled polypropylene and aluminum oxide powder filledpolyethylene.
 7. The device in accordance with claim 1 wherein the firstinsulating layer is of cross-linked polypropylene mixed with aluminapowder and the second insulating layer is of sulfur hexafluoride gasunder pressure greater than 1 atm.
 8. The device in accordance withclaim 1 wherein the first insulating layer is of cross-linkedpolyethylene mixed with alumina powder and the second insulating layeris of sulfur hexafluoride gas under pressure greater than 1 atm.